The present invention relates to heater control systems for controlling the electric current or power to be supplied to a heater, and more particularly to such a heater control system for use in the fixing unit of an image forming apparatus.
Usually, the resistance value of heaters is small when the heater is at room temperature immediately after energization and increases as the temperature of the heater rises. For example, a 1200-watt heater has a resistance value of about 0.8 ohm when at room temperature, and the value increases to about 8.3 ohms when the heater temperature rises to the steady-state level. Accordingly, an excessive rush current flows through the heater immediately after energization. In the case of the 1200-watt heater, the rush current is about 120 A. The rush current is therefore likely to cause welding of the contact of the relay for controlling the heater temperature or a malfunction of the power supply.
To overcome this problem, Unexamined Japanese Patent Publication SHO 63-310587 discloses a system which is adapted to connect a resistor in series with a heater only when the heater temperature is low and to short-circuit the resistor when the heater temperature has risen to suppress the rush current immediately after the closing of the power supply and eliminate the influence of the ohmic loss.
However, if the resistor of the heater control system is set, for example, to 7.5 ohms to suppress the rush current to the current (about 12 A for the 1200-watt heater) at the time when the heater is heated, the ohmic loss is 1080 W. Accordingly, the resistor has a large size and evolves heat due to the resistance. Further because the rush current is suppressed only by the resistor, the current through the heater decreases as the resistance value of the heater increases with a rise in the heater temperature.
On the other hand, heaters for use in the fixing units of copying machines and like image forming apparatus are on-off controlled so as to be maintained in a predetermined temperature range (between the fixing temperature and a lower temperature limit) and to ensure an improved image quality and reduced power consumption (Unexamined Japanese Patent Publications SHO 55-108678 and SHO 55-79477).
More specifically, the heater control system of the publication SHO 55-108678 assures savings in power by on-off controlling the heater for temperature control while the apparatus is in a stand-by state for copying operation and contolling the heater output proportionally or nearly stepwise during the copying operation.
The heater control system of the publication SHO 55-79477 is adapted to apply the total voltage to the heater of the fixing roller until the fixing temperature is reached to raise the surface temperature of the fixing roller at an accelerated rate, to on-off control the heater for temperature control after the fixing temperature has been reached, and to turn off the heater when the exposure lamp is turned on so that the power to be used will not exceed the rated power.
Nevertheless, the heater control system of the publication SHO 55-108678 requires much time to raise the heater temperature to the fixing temperature since the power supply to the heater is suppressed to reduce the heater output for savings in power.
Although the control system of the publication SHO 55-79477 rapidly raises the heater temperature since the total voltage is applied to the heater until the fixing temperature is reached, the heater temperature drops while the exposure lamp is on since the heater is off during this period. Accordingly, it takes some time for the heater temperature to reach the fixing temperature after the exposure lamp is turned off. Further because the heater is off while the exposure lamp is on, the power used is well below the rated power, with the result that the overall apparatus uses power with a reduced efficiency.